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Thirty-five years ago, Washington launched a program to minimize the civilian use of highly enriched uranium (HEU)—a terrorist’s material of choice for constructing nuclear weapons. Though the program struggled for funding, nongovernmental organizations and several Congressional champions kept it alive, and in 2004, attempts to reduce HEU stores began to blossom. That’s when the National Nuclear Security Administration launched the Global Threat Reduction Initiative, which works to remove weapons-grade fuel and convert HEU-powered research reactors to reliance on less-enriched fuels. In 2009, these efforts received a boost when US President Barack Obama announced a new goal to lock down all “vulnerable nuclear material around the world in four years” and Washington launched a series of head-of-state-level meetings, the Nuclear Security Summits.

Programs to minimize the civilian use of HEU will soon reach an important milestone: the near-completion of removals, under which governments repatriate fresh and spent HEU fuel to the country where it was originally produced, usually Russia or the United States, for eventual disposition. Obama’s ambitious four-year goal is unlikely to be met, but by October 2014, the Global Threat Reduction Initiative plans to complete 85 percent of all Russian-origin removals, 80 percent of all US-origin removals, and 89 percent of other (so-called “gap material”) removals. Thus, with more than a year and a half to go before the 2016 Nuclear Security Summit, all but 16 percent of the total 5,244 kilograms of targeted materials will have been repatriated. This success, in turn, will present a new challenge: Whereas it was relatively easy to track the removal of nuclear materials, it’s not clear how policymakers will measure nuclear security progress in the future.

The goal of global nuclear security involves more than just removal of weapons-usable material from civilian operations. Nations participating in the Nuclear Security Summit process also strive to counter smuggling and improve physical security at nuclear facilities. But removing civilian HEU has been the most tangible success to emerge from the US-led effort.

That’s partly because removals provide the easiest way to grasp the geographic scope of the nuclear terrorism threat and measure progress in reducing it. HEU removal is tracked with several hard metrics: kilograms of materials removed, the number of countries cleaned out completely, and the number of bombs that could have been made from the materials removed. The most recent removal update from the Global Threat Reduction Initiative offers an example: “To date, [the program] has removed more than 3,600 kilograms of HEU and plutonium from more than 40 countries around the world and has removed all HEU from 24 countries.” According to the Initiative’s most recent fact sheet, the materials removed were “enough for more than 140 nuclear bombs.”

As the nature of removal operations has become more complex, the metrics of HEU removal have remained simple—to a fault. The publicized total of 3,600 kilograms covers a variety of different materials, including fresh and spent HEU fuel repatriated, fresh and spent HEU fuel disposed of in-country, and plutonium. Not all of this material is equally dangerous. For example, out of 1,960 kilograms of Russian-origin HEU repatriated between 2002 and 2013, only 37.5 percent (or 739 kilograms) of the total was fresh HEU fuel, of which a relatively small amount is needed to improvise a nuclear device.

Such faults aside, the hard metrics of removal are powerful—after all, even one bomb is enough for an act of nuclear terrorism and even that may be one too many. Plus, removal makes for good drama. Typically photographed or recorded on video, removal operations have become the most transparent aspect of threat reduction. Upon their public release, images of cranes loading casks containing weapons-usable materials onto cargo planes, trains, ships, or trucks offer striking evidence of progress.

An additional advantage of HEU removal is that in the Nuclear Security Summit process, a pledge to minimize or eliminate weapons-usable materials makes for a nice “house gift”—that is, a tangible success that a nation can announce it has achieved. The house gift approach, introduced at the 2010 Summit, has encouraged governments to work towards civilian HEU minimization by giving them high-level credit for cooperation. As a 2013 nongovernmental report noted, since the Seoul summit in 2012, 18 out of 22 participants that possess at least a kilogram of HEU have either announced plans or taken actions to minimize usage, repatriate fuel, and convert research reactors.

As removals through the Global Threat Reduction Initiative slow to a trickle, though, governments will be left with fewer tangible ways to demonstrate their commitments and assess progress. For instance, it’s hard to measure the impact of efforts to combat smuggling because the problem’s scope is hard to define, and the data self-reported by countries varies in consistency and reliability. This, in turn, makes it difficult to establish the effectiveness of programs that provide training and equipment. Similarly, evaluating efforts to improve physical security and strengthen national regulation requires a near-impossible comparison across countries. Finally, while it’s important to increase the number of nuclear security-related exercises, courses, and workshops, the fact that they’re happening offers at best an imprecise estimate of improvement.

The next Nuclear Security Summit, which will take place in the Netherlands in March 2014, provides states with an opportunity to begin the transition from hard metrics to less tangible, “soft” approaches. Governments can start by developing a consistent way to report on their nuclear security efforts. They should also publicize information that is now treated as confidential or classified. Such information-sharing would allow other observers, like scientists and policy experts at non-governmental organizations, more opportunities to help measure progress. After all, preventing nuclear terrorism is a burden that governments shouldn’t have to shoulder alone. Without such shifts toward soft metrics, the very leaders trying to reduce the threat of nuclear terrorism will find themselves increasingly in the dark.

Editor's note: Anya Loukianova, program officer at the Stanley Foundation, wrote this column.